On your budget I would say the 4870 is out of the question, the reason I am saying that is because I am pushing you to get the 8 series Core 2 Duo processor over the 4 series. It costs about double but is well and truly worth it. I am building this PC with the 4850 graphics card, which is a very very good performer for the cash, you will see more benefit from the 8400 processor and the 4850 card than you would a lower 4 series processor and the 4870 card.
These 8 series processors are loaded with cache which is perfect for gaming, they are probably the best gaming processor on the market, and at just over £100 at throw are at an amazing price also. Also the higher bus speed is a massive boost when coupled with a 1333mhz board, whereas the e4 series are 800mhz, (I think, never checked for ages.)
http://www.ebuyer.com/product/145527
Gecube HD 4850 512MB GDDR3 Dual DVI HDCP HDTV Out PCI-E Graphics Card
£115
http://www.ebuyer.com/product/139971
Intel Core 2 Duo E8400 Socket 775 (3.0GHz) 1333FSB 6MB L2 Cache Retail Boxed
£128
http://www.overclockers.co.uk/showp...SLi (Socket 775) PCI-Express DDR2 Motherboard
**B Grade** MSI P7N Diamond nForce 780i SLi (Socket 775) PCI-Express DDR2 Motherboard
£60
http://www.ebuyer.com/product/131214
Maxtor STM3250310AS 250GB SATAII 8MB Cache - OEM
£28
http://www.ebuyer.com/product/129177
Kingston 2gb Kit (2x1gb) Ddr2 800mhz/pc2-6400 Hyperx Memory Non-ecc Cl4
£29
http://www.ebuyer.com/product/109552
Hiper 580W Type-M Black PSU - SLI Certified with APFC
£38
Add your own case and your away, obviously you may need kayboard, mouse and speakers, but that again is personal preferance.
I chose that motherboard from Overclockers because it is an awesome board at a wicked price, you seriously will not get a better deal on this board anywhere else. You may need to purchase your SATA cables separate and maybe a back plate, but taking that into consideration it is peanuts considering the price you are paying for one of the best boards on the market.
Also I have gone for the Hiper PSU as I can't seem to find the OCZ 500w, so this will do, not quite as good quality but they are good enough while trying to keep costs down.
Total is £398, then you need your case and mouse and stuff.
A great buy in my eyes.
EDIT:
Also add a DVD burner for about £15, one to match a case of your choice. Overall if you need keyboard and mouse and such I think the whole build will touch about £450, but well worth it mate.